The kappa opioid receptors (KORs) have been implicated in a number of psychiatric disorders including depression and related mood disorders, anxiety and stress-related disorders, addictions such as cocaine and other psychostimulant dependence, alcoholism, and Alzheimer's disease. The availability of PET imaging agents will provide non-invasive biomarkers to interrogate the KOR and gain insights into the function and dysfunction of this receptor system in the aforementioned disorders. Until recently there were no validated PET radiotracers to image KOR in humans, although radiotracers for other opioid receptor subtypes have been available for some time. In the last year we evaluated in humans two KOR ligands, one agonist and one antagonist, as PET tracers. The antagonist tracer, [11C]LY2795050, proves to be suitable for imaging and quantifying KOR in the human brain. However, the agonist tracer, [11C]GR103545, is not ideal, due to its slow tissue kinetics. The validation of [11C]LY2795050 marks the first time a KOR-selective tracer is available for use in humans. In this combined R21/R33 Phased Innovation Award application, we propose to build upon our expertise and experience in the development and evaluation of KOR PET tracers, and to synthesize, evaluate and validate two C-11 and/or F-18 labeled KOR agonist radiotracers for PET imaging studies in humans. Specifically, in the R21 phase of investigation we will implement a medicinal chemistry discovery, in vitro binding and functional assays, and in vivo PET imaging screening evaluation of candidate agonist tracers in non-human primates to critically appraise and select two C-11 and/or F-18 labeled tracers (and ideally, one C- 11, and one F-18 labeled tracer) for progressing to the R33 phase of comprehensive tracer evaluation and validation studies. In the R33 phase of investigation, the R21 selected tracers will undergo first a comprehensive evaluation in non-human primates to confirm in vivo binding specificity and selectivity, and subsequent quantitative PET imaging studies in humans to quantify regional specific binding signals, to assess the reproducibility of binding parameters and to determine non-displaceable volume of distribution. The ultimate goal is to provide the PET imaging community with a pair of agonist-antagonist radiotracers for the kappa opioid receptors. The development, and successful deployment of such an optimal, effective agonist-antagonist imaging tracer pair will enable, for the first time, the in vivo investigation of not only KOR expression, but also its functional state, in a variety of neuropsychiatric disorders and addictive conditions.